Tuning Holley four-barrel carbs

First of a double feature on tuning Holley’s hot street four-barrels.

Hib Halverson - September 16, 2011 12:00 PM


The 850 Holley of today is similar to the one covered in this article. It differs mainly in its metering block and float bowl designs. Image: Holley Performance Products.


You can buy a fancy carb stand or you can make your own with a couple of bucks worth of 5/16-inch bolts, nuts and washers.


If you’re going to work on Holleys, buy a jet tool. It’s designed to tighten and loosen jets without damaging them. Use one of these and your jets will love you.


The most important tool we used in this two-part series: the Auto Meter “Wide-Band Air Fuel Ratio Monitor”. We mounted it on the steering column. This was what we saw during an early test of the car’s highway fuel economy. We’re just on the rich side of stoichiometric.


The Auto Meter wide-band O2S comes with everything you need for installation – Bosch sensor, sensor bung, wiring harness and instructions. Have the bung welded into your exhaust. We had ours put just downstream of the passenger side header collector.


The baseline adjustment for float bowls with sight glasses is halfway up the glass. Shown is the secondary bowl on our 850.


Our metering blocks (primary shown) have jet extensions (p/n 122-5000), vent “whistles” and large-head idle mixture screws (p/n 26-101). The extensions prevent the secondary jets from “starving” for fuel under hard acceleration and the primary jets from starving under hard braking. The vent whistles, standard on double-pumpers, prevent fuel from sloshing out the vents during aggressive driving. My little finger is partially covering one of the large-head idle mixture screws, which are easier to access with a screwdriver.


Adjust your primary idle speed screw so, when you look at the top of the primary throttle plate, you can see .020 to .030-inch of the idle transfer port.


We chose to remove the plates for drilling but it can be done with the plates on the throttle shafts. Clean the throttle body of any chips or metal dust before you reassemble the carb.


The bypass holes need to be between the shaft and the barrel’s idle transfer port. If you’re really a pro, after you finalize your bypass hole size, give each hole a slight chamfer.


Pros adjust idle mixture using a vacuum gauge. Look for the highest vacuum at which the engine runs smooth.


Installing wire “V”s into the primary metering block takes good eyesight and “pixie fingers.” We had to use a magnifier and a pair of tweezers. You need two “V”s, one for each primary idle feed restriction.


This chart shows air-fuel ratio ranges for various engine performance regimes. Pink is the range of AFRs for best torque output. Red is the range for maximum power. Green is the range for a combination of low exhaust emissions and good fuel economy and blue is for best gas mileage. Image: Toyota Motor Sales USA

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Before you begin, make sure your ignition system is in good condition.

I got my old hot rod back in 1980 in a smokin’ deal from a fellow car club member. Here I was – a kid with an affinity for burn-outs who’d hit the big time: a light car with a limited-slip diff, 255/60 tires and a big-block engine. Luckily, I didn’t get in trouble (well, not too much) and, boy, did I have fun!

By the mid-’90s, the engine got boring, so I modified that 460-inch V-8 with a steel crank, Crower rods, 9.75:1 Bill Miller Engineering forged pistons, Crane mechanical-lifter roller cam (.567-inch lift, 234°/244° duration at .050-inch), DeGroff ported/polished heads with bigger valves, Crane dual valve springs, Crane roller rocker arms, an Edelbrock Performer RPM intake, Doug’s Headers, MSD 6 ignition, a McLeod Racing aluminum flywheel and a McLeod “Street Twin” clutch. The engine makes about 550hp at 6,000 to 6,200 rpm. I shift the car’s Richmond “ROD” six-speed trans at 6,400 rpm. The rev limiter is at 6,750.

Sitting on the Performer RPM intake is an 850cfm Holley – a classic, model 4150, “Double-Pumper” four-barrel carb. “DPs” come in five cfm ratings, 600 to 850. They have mechanical secondaries, dual accelerator pumps on the 750 and 850, four idle mixture screws (“four-corner idle”), and a manual choke. Holley bills them as “... performance upgrades for hot street cars and race vehicles.”

When their flow rating is matched to a car’s powertrain and weight, in a racing application, Double-Pumpers run OK right out of the box, but what about part throttle – as in normal street driving? Because stock DP calibration is intended for racing, their driveability may leave something to be desired, as may their fuel economy.

Modifying Holley double pumpers for “double duty” – i.e. performance and better driveability – is a craft practiced by vintage road racers, NASCAR crew chiefs and finicky hot rodders. Mastering this fusion of art and science is time-consuming, sometimes tedious work, but is, also, refreshingly “old-school”.


Setting the Stage

My 850 (p/n R4781-5) dates to the mid-1990s, but Holley still makes the 850 double-pumper in a slightly different version (p/n R4781-8). It’s recommended for engines of about 450 cubic inches with large (but not “huge”) intake port volumes, a big cam profile, headers and an rpm range of 2,000 to 6,500. It’s suggested for cars with short gears and either manual transmissions or automatics with high-stall converters.

In the ’90s, the initial mods to my 850 were by the now-defunct Fuel Curve Engineering. To improve fuel atomization and enhance low speed response, annular discharge boosters were installed in the primary barrels. Annular boosters are a slight restriction at high airflow, but I’ll take that hit in exchange for better response.

The secondary barrels were polished and fitted with Fuel Curve Engineering boosters. To bridge the point where the secondary idle transfer circuit is saturated but the main circuit is not fully engaged, an intermediate circuit was added. Some believe this makes mechanical secondaries work better on the street. Others, such as Bob Verbancic, who runs The Carb Shop, a speciality shop in Ontario, California, say that today’s booster designs, coupled with proper main circuit calibration and accelerator pump choices, makes three circuit secondaries unnecessary. Since changing boosters and a new secondary metering block aren’t in the budget right now, I’m sticking with the intermediate circuit.

A choke is suggested for a double-pumper calibrated for street use because the part-throttleair/fuel ratio of 14.5 to 15.5:1 needed for better gas mileage can make cold starts difficult. I like Holley’s electric choke kit (p/n 45-224) because it eliminates fooling with a manual choke. To reduce restriction of airflow into the primary barrels in applications requiring a low air filter top due to limited hood clearance, the choke horn was milled to the minimum necessary to seal the choke plate.

Because special tools and a mill are required, modifications we’ve covered so far are seldom do-it-yourself operations and we suggest The Carb Shop for the work.


Information and Tools

The advanced carb work discussed in this series requires an understanding of carburetion theory and function along with in-depth knowledge of Holley 4150 fuel metering systems. If you’re a rookie tuner, read Super Tuning and Modifying Holley Carburetors from CarTech Books, along with Holley 4150 & 4160 Selection, Tuning and Repair and the Holley Performance Catalog, all available from Holley. One source of Internet information is Holley’s streaming video site, If you’re good with Google, you’ll find more.

Finally, make friends with a Holley retailer, such as The Carb Shop, because you’ll need jets, power valves, accelerator pump cams and shooters, gaskets, a rebuild kit or other parts.

You need some special tools as well. A Holley jet tool (p/n 26-68) prevents damage to jets during installation or removal. A vacuum gauge is needed to set idle mixtures and select power valves. Tweezers or forceps are great for handing tiny parts. Very small drills, from #80 (.0135-inch) to #40 (.098-inch), may be necessary. You can’t use a drill motor with these tiny bits, so get a “pin vise,” a hand-operated tool for drilling small holes.

Accurate measurement of air/fuel ratio (AFR) is required. It’s done by an oxygen sensor (O2S) screwed into the exhaust. Cheap “rich-lean” indicators, which use narrow-band sensors, are inadequate. A modern, wide-band oxygen sensor is required to tune a double pumper’s driveability. I installed the Auto Meter “SportComp Wide-Band Air-Fuel Ratio Monitor” (p/n 3378) which I picked because: 1) it complemented the car’s existing SportComp instrumentation. 2) It uses a Bosch wide-band O2S with a resolution of 0.1 air-fuel ratio and 3) I like its combination of a numeric display (reads AFR or lambda) and a “psuedo-analog” display of colored LEDs.

The most important tool we used in this two-part series: the Auto Meter “Wide-Band Air Fuel Ratio Monitor”. We mounted it on the steering column. This was what we saw during an early test of the car’s highway fuel economy. We’re just on the rich side of stoichiometric.

Before you begin, make sure your ignition system is in good condition. If there are problems with it, address them before getting into carb tuning. Lastly, you’ll spend a fair amount of money on road testing. We burned hundreds of bucks worth of fuel in road testing our drivability mods. It’s possible to do the part-throttle work on a chassis dyno provided it’s a “brake dyno” which can load the car to constant speeds at part-throttle. That may cost as much or more because dyno time can be pricey. Any way you cut it, be it parts or testing; advanced carb tuning ain’t cheap.


Getting Started

The first change I made was to a set of stainless steel carb studs, nuts and washers (p/n 400-2403) from Automotive Racing Products (ARP). The ARP studs have rounded ends, which make carb removal/replacement easier. ARP’s stud kit is designed for spacers up to ½-inch high so it works with the Holley heat shield assembly (p/n 108-70).

Next came Holley’s aluminum float bowls (p/n 124-175, 134-175) which are lighter and have sight glasses rather than plugs for setting float level. Along with less weight and less messy float adjustments, I got a snazzy, polished aluminum finish. Sweet! Not only does proper float level prevent flooding or starving for fuel, it affects timing of the main circuit. You must set the float level with the car on a level surface. Aluminum bowls require a different adjustment procedure than bowls with sight plugs. Base float level should be halfway up the sight glass. If you have bowls with plugs, set the float level at the bottom of the holes. The secondary bowl of our 850 is shown on page 68.

A 4150 uses two circuits to control part-throttle air/fuel ratio: “idle transfer”, which starts just off-idle and provides fuel at low part-throttle and light load, and “main”, which influences medium part-throttle AFR at light-to-medium loads. Initial road testing with the Auto Meter wide-band showed part throttle AFR was way rich and our accelerator pump calibration – we’ll cover that in Part 2 – was too rich, as well. No wonder the car was a gas-guzzling pooch around town.

The first step in solving this problem was to optimize the relationship between the primary throttle plates and the primary idle transfer ports. If that’s not right, you may have trouble adjusting idle mixtures, throttle response problems and may make incorrect choices in idle feed restriction sizes and accelerator pump calibration.

Remove the throttle body. Adjust the primary idle screw so the first 0.020 to 0.030-inch of the primary idle transfer slots are visible. Reassemble the carb, run the engine and get the idle mixture screws close. If idle is too high, the fast idle cam might be stuck or there is a vacuum leak. Address those issues then try again. If idle speed is a little too low, adjust the speed screw a small amount – say 1/3 turn or so – until you have the speed you want. If idle speed is a lot too low, pull off the throttle body and drill bypass holes in each primary throttle plate to allow more air through the primaries without disturbing the relationship between the plates and transfer ports. Using your pin vise, drill the plates on the same side of the throttle shaft as the transfer ports. Start with a 1/16-inch hole in each plate and work up a drill number at a time until idle speed is about what you want. You can fine adjust the idle speed screw, however, more than a third-to-half a turn either way might have the plate/slot relationship out of whack and you’ll have to increase the bypass hole size. If you drill the holes too large, plug them with JB Weld epoxy and redrill smaller. Durability? I ran epoxied/redrilled plates for 20 years with no problems. So they’d look pretty in photos, I ordered new throttle plates (p/n 26-97) from The Carb Shop and drilled them right this time.

With four-corner idle carbs, the bypass hole process is more involved. With an idle circuit for each barrel, for best idle stability and drivability, idle speed and mixture adjustments are made equally to primaries and secondaries. Adjust the primary throttle plates so you see .020 to .030-inch of the idle transfer ports. Hold the throttle body up to a light and adjust the secondary stop screw so the secondary plates are open the same amount. The location of the transfer ports in the secondaries is different than in the primaries, so the bottom of the secondary transfer ports may not be visible. This will be easier if you install a Holley Secondary Adjusting Kit (p/n 26-117), which adds a conventional idle speed screw to the secondaries.

After running the engine, if you need bypass holes, start with a #70 bit (.028-inch) and drill all four plates on the side of the throttle shafts closest to the transfer ports. The holes in our 850 ended up .043-inch, on the low side of what can be needed. Generally, the more camshaft you have in the motor, the bigger the bypass holes need to be. Now, do an idle mixture adjustment using the vacuum gauge and your ear. Take each screw a quarter turn lean and listen to the engine and watch the vacuum gauge. If the engine runs rough or the vacuum drops, go back the other way. If the engine smoothes and vacuum increases, go another quarter turn lean. You want the highest vacuum level you can get. On the wide-band, the air/fuel ratio will vary depending on your camshaft profile, spark timing and engine configuration. I was able to lean my idle to the high 14s/low 15s.


Smooth Transfer

Coming into this tuning session, main jetting was 72 primary and 78 secondary. I dropped the primary mains five numbers to 67, then road tested with the vacuum gauge and the Auto Meter wide-band. My goal was, at 2,500 to 4,500 rpm with the engine loaded between 10 and 7 psi of vacuum (just above power enrichment at 6.5 psi), to have the air/fuel ratio between 13.5:1 and 15.5:1 and I was close enough for now.

Now, get the primary idle transfer mixture in the ballpark. With double-pumpers on street/track engines having modest duration/high lift roller cams typical of hot street engines, it will be too rich. Fuel is metered to the idle screws and the idle transfer ports by “idle feed restrictions” (IFR), little tiny jets which are not removable. You can’t make IFRs smaller, so the DIY fix is to restrict them with small wire “V”s. You’ll need either a good set of dial calipers or a 0 to 1-inch vernier micrometer to measure their diameter.

A good source of wire are the strands of automotive electrical wire, the diameters of which vary greatly. I’ve found as small as .008-inch and as large as .025-inch. Another source for larger sizes are straight pins – find a friendly tailor or seamstress who’ll give you some, snip off the ends and bend them into Vs. Medical forceps or small needle-nose pliers may be necessary to manipulate these tiny wires. I started with .015-inch wires, which in a .0355 primary IFR is a significant reduction in fuel flow. That was a pretty good guess as the light-throttle air/fuel ratio I saw on the Auto Meter went from low 12s to high 13s. With the air/fuel ratio headed in the right direction, I tried .0175-inch wire. That got me to the mid 14s, right around stoichiometric, where good fuel economy occurs. Now, my engine still “felt” good at cruise and was capable of better gas mileage. My idle feed wires are on the high side of what most double pumpers will need. Once the wires are in the IFRs, redo the idle mixture adjustment.

One caveat about leaning the part throttle mixture: you may find that if you did your idle and part throttle calibrations in warm weather, in cool and cold weather, the air/fuel ratio will be too lean due to the dense air. That may force you into changing the IFR wires periodically, which is no more difficult than changing jets. Nevertheless, if you don’t want that hassle, do your part-throttle calibration in cold weather and live with the more rich mixture when it warms up.

We’ll finish off our Holley double-pumper tuning session with main jetting, power valve selection and accelerator pump tuning. Click here for Part II.